1. Field of the Invention
The present invention generally relates to a wireless communications system including multiple transmitting stations and a receiving station for receiving information from the transmitting stations, as well as to a communication control technique used in such a wireless communication system.
2. Description of the Related Art
In wireless communications systems (such as mobile communications systems), contention occurs in a channel when multiple transmitting wireless stations have to share the channel to transmit data simultaneously. Accordingly, some access protocol for allocating the channel to each of the transmitting wireless stations is required to prevent data collisions. JP 11-196473A discloses a technique for avoiding collisions that may occur when multiple mobile stations power on at the same time. In this publication, a unique number is assigned to each mobile station. By counting down the number at each mobile station upon power-on, location registration signals are transmitted from the mobile stations at different timings.
Conventional access protocols are grouped into collision-free scheduled access protocols and random access protocols that may accompany data collision. An example of the scheduled access protocol is time division multiple access (TDMA) conventionally used in cellular communications system. TDMA is a digital transmission technology that allows multiple users to access a single radio-frequency (RF) channel without interference by allocating unique time slots to each user within each channel. However, since only a limited number of time slots are available in a single channel, appropriate transmit control becomes difficult under the situation where a number of cellular phones exist.
Random access protocols include repeat-type random access, such as ALOHA or S-ALOHA, and reserve-type random access, such as r-ALOHA. With ALOHA protocols, a transmitting wireless station waits a random amount of time and repeats transmission when data collision occurs. In this case, the receiving wireless station has to wait continuously until the data are correctly received. This scheme is inefficient because of large power consumption.
FIG. 1A illustrates an example of conventional periodic multiple access, in which all the transmitting wireless stations transmit periodically and the receiving wireless station performs intermittent receive. FIG. 1B illustrates an example of conventional random access, in which all the transmitting wireless stations transmit data after a random amount of time and the receiving wireless station performs continuous receive.
FIG. 2 is a schematic block diagram of a transmitting wireless station that employs a conventional periodic multiple access protocol, and FIG. 3 is a schematic block diagram of a receiving wireless station that employs a conventional periodic multiple access protocol.
The conventional transmitting wireless station 100 shown in FIG. 2 has a data storage unit 101, an isochronous burst transmission control unit 102, a clock generating unit 103, a transmitting unit 104, a switch 105, a power ON/OFF unit 106, and an antenna 107.
The data storage unit 101 stores transmit timing signals (Hnum), the number of slots in a single time frame (Fslot), time frame (Ftime) information, transmit clock information (clock_t), and other necessary information. Transmit timing signal (Hnum) represents the slot number assigned to the transmitting wireless station 100 for constant-interval data transmission. The number of slots (Fslot) represents how many slots are provided in a single time frame. A time frame (Ftime) represents a single frame of time required for one-frame transmission. The transmit clock information (clock_t) represents a time clock at the transmitting wireless station 100.
The clock generating unit 103 counts clocks, which are time information required to perform burst transmission of data. The maximum value of the counter is consistent with the number of slots (Fslot) in a single time frame. The clock generating unit 103 outputs the counter value as the transmit clock information (clock_t) any time to the isochronous burst transmission control unit 102, and simultaneously, stores the counter value in the data storage unit 101.
The isochronous burst transmission control unit 102 reads a transmit timing signal (Hnum) from the data storage unit 101, while it receives the transmit clock information (clock_t) from the clock generating unit 103. Whenever the transmit timing signal (Hnum) and the transmit clock information (clock_t) are the same, the isochronous burst transmission control unit 102 supplies a switch-on instruction to the switch 105. When the transmit timing signal (Hnum) and the transmit clock information (clock_t) are not consistent with each other, the isochronous burst transmission control unit 102 does not supply the switch-on instruction.
The transmitting unit 104 is connected to the antenna 107 via the switch 105 to transmit the input data. When the switch 105 receives the switch-on instruction from the isochronous burst transmission control unit 102, the data from the transmitting unit 104 are transmitted from the antenna 107. In this manner, the transmitting wireless station 100 transmits data at a constant interval (periodically). The switch 105 is in the off state unless it receives the switch-on instruction from the isochronous burst transmission control unit 102.
The power on/off unit 106 turns on or off the respective circuits in the transmitting wireless station 100 in response to a power on/off instruction supplied externally.
On the other hand, the receiving wireless station 110 shown in FIG. 3 includes a data storage unit 111, a receiving unit 112, an isochronous burst receive control unit 113, a clock generating unit 114, a power on/off unit 115, and an antenna 116.
In the data storage unit 111 are stored receive timing signals (Hnum_r), the number of slots in a single time frame (Fslot), time frame (Ftime) information, receive clock information (clock_r), etc. Receive timing signal (Hnum_r) represents the slot number of a slot having arrived from the transmitting wireless station 100. The receive clock information (clock_r) represents a time clock at the receiving wireless station 110.
The clock generating unit 114 counts clocks, which are time information required to perform intermittent receive of data. The maximum value of the counter is consistent with the number of slots (Fslot) in a single time frame. The clock generating unit 114 outputs the counter value as the receive clock information (clock_r) any time to the isochronous burst receive control unit 113, and simultaneously, stores the counter value in the data storage 111.
The isochronous burst receive control unit 113 reads a receive-timing signal (Hnum_r) from the data storage unit 111, while it receives the receive clock information (clock_r) from the clock generating unit 114. Whenever the receive-timing signal (Hnum_r) and the receive clock information (clock_r) are consistent with each other, the isochronous burst receive control unit 113 supplies a receive instruction to the receiving unit 112. When the receive-timing signal (Hnum_r) and the receive clock information (clock_t) are not consistent with each other, the isochronous burst receive control unit 113 does not output the receive instruction.
Upon acquiring the receive instruction from the isochronous burst receive control unit 113, the receiving unit 112 receives data from the transmitting wireless station 100 via the antenna 116. The receiving unit 112 performs a CRC check on the received data. If there is no error in the received data, the receiving unit 112 outputs the data externally. The power on/off unit 115 turns on or off the respective circuits in the receiving wireless station 110 in response to a power on/off instruction supplied externally.
FIG. 4 is a schematic block diagram of a transmitting wireless station that employs a conventional random access protocol, and FIG. 5 is a schematic block diagram of a receiving wireless station that employs a conventional random access protocol.
The transmitting wireless station 200 shown in FIG. 4 has a data storage unit 101, a clock generating unit 103, a transmitting unit 104, a switch 105, a power ON/OFF unit 106, an antenna 107, a random number generator 201, and a burst transmission control unit 202. The same elements as those shown in FIG. 2 are denoted by the same symbols, and explanation for them is omitted.
The random number generator 201 reads the number of slots (Fslot) of a single frame from the data storage unit 101 to generate random numbers taking values of 1 through Fslot, and stores the random numbers as the transmit timing signal (Hnum) in the data storage unit 101. The random number generator 201 also reads the transmit clock information (clock_t) from the data storage unit 101. If the transmit clock information (clock_t) is zero (0), a new random number is generated and stored as updated transmit timing signal (Hnum) in the data storage unit 101.
The burst transmission control unit 202 reads the transmit timing signal (Hnum) from the data storage unit 101, while it receives the transmit clock information (clock_t) from the clock generating unit 103. Whenever the transmit timing signal (Hnum) and the transmit clock information (clock_t) are consistent with each other, the burst transmission control unit 202 supplies a switch-on instruction to the switch 105. Upon receiving the switch-on instruction from the burst transmission control unit 202, the switch 105 allows the data from the transmitting unit 104 to be transmitted from the antenna 107. The switch 105 is in the off state unless it receives the switch-on instruction from the burst transmission control unit 202.
On the other hand, the receiving wireless station 210 shown in FIG. 5 includes data storage unit 111, a receiving unit 112, a clock generating unit 114, a power on/off unit 115, and an antenna 116. The same elements as those shown in FIG. 3 are denoted by the same numerical references, and explanation for them is omitted. The receiving wireless station 210 does not perform intermittent receive, unlike the receiving wireless station 110 shown in FIG. 3, and instead, it is always in the waiting state to perform continuous receive.
However, with the conventional periodic multiple access protocol, the transmit timing of the transmitting wireless station i and the transmit timing of the transmitting wireless station j overlap each other, as illustrated in FIG. 1A. Accordingly, once data collision occurs, data collision keeps on occurring in the subsequent time frames. With the conventional random access protocol, the receiving wireless station has to always be in the waiting state because the transmit timing of each transmitting wireless station is unknown. This causes the power consumption to increase.